Semiconductor crystals of fibrous structure and method of their manufacture



m: eg 429s? May 6, 1969 MULLER ET AL 3,442,823

SEMICONDUCTOR CRYSTALS OF FIBROUS STRUCTURE AND METHOD OF THEIR MANUFACTURE Filed Oct. 22, 1965 Sheet of 3 Fig.2

May 6, 1969 MULLER ET AL 3,442,823

SEMICONDUCTOR CRYSTALS FIBROUS STRUCTURE HOD OF TH AND MET MANUFACTURE Filed Oct. 22, 1965 Sheet 2' of s y 6 1969 A. MULLER ET AL 3,442,823

SEMICONDUCTOR CRYSTALS OF FIBROUS STRUCTURE AND METHOD OF THEIR MANUFACTURE Filed Oct. 22, 1965 Sheet 3 Of 3 Fig.7

United States Patent US. Cl. 252-518 7 Claims ABSTRACT OF THE DISCLOSURE Described is a polycrystalline semiconductor body consisting of fibrously elongated crystallites having substantially parallel longitudinal directions, said fibrous crystallites being individually monocrystalline and coalesced to one another and having substantially all the same crystallographic orientation in the longitudinal direction.

Also described is the method of producing a polycrystalline semiconductor body. This method comprises melting a mass of semiconductor material having the shape of an elongated body, freezing the body progressively from one end to the other, fusing to the last-frozen end a polycrystalline seed having a fibrous texture formed by monocrystallites Whose longitudinal directions are substantially parallel to that of the body, and zone melting the body starting at the seed to impart a similar fibrous texture to the resulting polycrystalline product.

Our invention relates to polycrystalline semiconductor bodies having a crystalline texture of fibrous character, each of the mutually coalesced fibers being constituted by a monocrystallite, i.e. a monocrystalline grain, of elongated shape whose longitudinal direction is substantially parallel to that of the other monocrystallites.

The semiconductor material required for investigating physical semiconductor properties or for producing electrical or optical semiconductor devices, must possess an accurately defined content of dopant atoms which are built into the lattice of the semiconductor crystal and perform donor or acceptor functions. In the following, a semiconductor crystal is designated as homogeneously doped if it does not possess macroscopic domains, for

example of the size or volume of individual crystallites or grains, having a higher dopant concentration than an adjacent other domain.

The dopant concentration in the known semiconductor polycrystals, produced in a simple manner by pulling them out of a doped melt and/or preparing them by zone melting, has been found to vary greatly from grain to grain of the polycrystal. This is due to the fact that the dopant substance becomes built into each granule in dependence upon the crystallographic orientation of this particular granule. In a polycrystal, as a rule, only granules or crystallites of the same direction of growth exhibit the same dopant concentration.

On account of the relatively uncomplicated manufacture available for polycrystals, there has been no lack of attempts at producing homogeneously doped semiconductor polycrystals. Past efforts in this direction, however, have failed to yield reliable or useful results. Furthermore, the production methods became so critical that it has been easier and less costly to pull monocrystalline semiconductors out of doped melts. Monocrystals, pulled with the aid of corresponding, carefully oriented seeds, have an accurately defined direction of growth and consequently satisfy the above-mentioned condition for homogeneous doping. However, this holds good only, partic- 3,442,323 Patented May 6, 1969 ularly if the monocrystals being produced are rather thick, if care is taken that during crystal pulling or zone melting the phase boundary between the solid and the liquid material is planar and extends perpendicularly to the direction of growth. Maintaining the desired planar geometry of the phase boundary is difiicult. Besides, the production of monocrystals encounters other difiiculties of considerable magnitude. This is because satisfactory monocrystals can be pulled only by observing accurately prescribed conditions of solidification, such as those relating to temperature gradient, pulling speed, orientation of the crystal seed, and absence of spurious seeds or contaminations within the melt. Minute departures from the optimal values may sufiice to result in crystalline disturbances, particularly the formation of twin crystals, since the activating energy of the twin structure is very slight.

Relative to the electrical properties of semiconductor monocrystals, however, the coalescence planes of twins constitute only very slight disturbances of the crystal. Indeed, a twin-growth of monocrystals fr-om semiconductor material has already been found applicable in electrical devices. Twin structures of monocrystals, for instance, are described in the German published patent application 1,179,645.

In principle, the pulling of a semiconductor polycrystal involves considerably lesser requirements with respect to equipment and processing than the pulling of a monocrystal, even if the monocrystal is of the twin type just mentioned. For example, with polycrystals the accurate orientation of a suitable seed, to be performed roentgenographically, may be dispensed with. No detrimental effect is encountered of any residual slag on the surface of the melt, as may result from frictional wear of the crucible material or may be caused by reaction of the melt with gaseous impurities in the inert gas atmosphere being used. Furthermore, the adjustment and maintenance of a favorable crystallization rate for given temperature conditions are less critical than for the pulling of monocrystals.

It is therefore an object of our invention to provide a semiconductor polycrystal which can readily be homogeneously doped during its production.

A further object of the invention is to provide a simple and industrially applicable method of producing polycrystalline semiconductor bodies of the" types mentioned above.

Another object of the invention is to devise two-phase crystalline semiconductor bodies which meet the increas ingly more exacting requirements of the electronic industry by being producible in a simple manner largely free from susceptibility to trouble, and in which the second or minor phase is constituted by electrically or magnetically good conducting inclusions which are substantially all oriented uniformly.

Still another object of the invention is to develop a method for the production of two-phase polycrystalline semiconductor bodies which results in a simple manner in the desired inclusion and parallel orientation of the second conductive phase.

A polycrystalline semiconductor body according to the invention consists of fibrous elongated grains or crystallites (fibers) whose respective longitudinal axes are substantially parallel to one another. These crystallites are monocrystals and possess the same crystallographic orientation in the longiutdinal direction. The fibrous monocrystals according to the invention may consist of any suitable semiconductor material such as silicon, germanium, or semiconductor compound, for example an A -B compound such as InSb, InAs.

According to another feature of our invention, the material of the fibrous polycrystalline semiconductor body consists of a two-phase material, the quantitatively pre dominant, main phase being consituted by semiconducting material and the second phase being constituted by geometrically anisotropic, electrically or magnetically conducting inclusions Which are distributed in the first phase in mutuallyspaced relation, the inclusions having their longitudinal axes extend preferably in the direction of the elongated fibers or crystallite s jn which they are embedded.

Referring to any of the abbve-mentioned fibrous polycrystals according to the invention and in accordance with a further feature of our invention, the individual crystallites or fibers within the polycrystalline semiconductor body may be constituted by coalesced monocrystalline twins-Whose plane of coalescence extends in the longitudinal direction of the crystallite.

As mentioned above, the longitudinal axes of the individual monocrystals within the polycrystalline body are to be substantially parallel to each other. This, however, may include a slight angular or skewed relation of the crystallities relative to each other, up to an angular departure of about from strict parallelism. As a rule, however, the directions of the crystallite longitudinal axes do not depart more than 1 to 3 from the median longitudinal direction obtained as an average from all of the crystallites within the polycrystal. In many cases a departure of 2 to 3 is found to exist.

As mentioned, a polycrystal according to the invention may be designated as a crystal of fibrous structure. Such a fibrous polycrystal is suitable as a seed for pulling homogeneously doped semiconductor crystals from doped melts. Since the longitudinal axes of the individual crystallites (fibers) of the then resulting polycrystal point all in the same crystallographic direction, they all possess the same dopant concentration.

Ordinary polycrystals consist of a coalesced bunch of randomly oriented and randomly shaped grains. In contrast thereto, the polycrystals according to the invention are formed of elongated fibrous grains or fibers. The longitudinal dimension of these monocrystalline fibers may be 1 cm. for example but, in general, is often much longer. Lengths of cm. and more have been measured. The fiber diameter varies between values in the order of magnitude of 1 micron and a few millimeters.

A characteristic exhibited by most of the fibers in a polycrystal according to the invention is their twin-type constitution. That is, the monocrystallites within the polycrystal have the tendency to consist each of coalesced twin laminations. The coalescence planes of the twins between each two of these laminations occur in one end and the same fiber with. only a single orientation. In other words, these planes are substantially parallel to each other. In different monocrystalline fibers within the same polycrystal, however, the coalescence planes of the twins may occur in greatly different numbers and at greatly different distances from each other, these distances, as a rule, being different in different crystallites respectively. On the average, each monocrystalline fiber contains about ten to fifteen twin coalescence planes; but there also occur fibers with no such plane or with only one or two planes, and fibers have also been found traversed by about one hundred or more coalescence planes. The spacing between successive planes of the twin crystallites may vary between fractions of 1 micron and a few millimeters, and such variation is observed within one and the same fiber as well as from fiber to fiber.

The longitudinal direction of the twin laminations whose coalescence planes are {11l}-planes extends perpendicularly to the l11 -direction appertaining to each coalescence plane. For example, one of these longitudinal directions is a crystallographic l10 -direction.

Relative to semiconductor crystals having a diamond lattice, particularly silicon and germanium, a {11l}- plane may be looked upon as being that of a. twin crystallite. The twin formation may also involve a 180 rotation of the one twin individual relative to the-other about a rotational axis constituted by a l11 -direction. The semiconductor crystals having a Zinkblende structure and hence possessing no symmetry center are in accordance with the twin-formation law last mentioned. This applies, for example, to the A B compounds of respective elements from the third and fifth groups of the periodic system.

Fibrous polycrystals according to the invention as well as methods of producing them will be described hereinafter with reference to the accompanying drawings in which:

FIG. 1 shows schematically an enlarged longitudinal section through a crystal according to the invention.

FIG. 2 is a cross section along the line IIII in FIG. 1.

FIG. 3 is a perspective and schematic view of a few fibers which form part of a semiconductor polycrystal according to the invention.

FIG. 4 shows schematically a device suitable for the production of such fibrous monocrystals. B

FIG. 5 shows schematically an enlarged longitudinal section through a two-phase crystal according to the invention with distributed inclusions of conducting material.

FIG. 6 is a cross section along the line VIVI in I FIG. 5; and

grain 3 terminated its growth at one spot of the illusstrated section. Another fibrous grain 4 commenced growing and has partially occupied the space of a fibrous grain 5 which extends all the way through the sectioned area.

It will be seen from the cross section shown in FIG. 2 how the coalescence planes of the twins 2 are rotated relative to each other about the longitudinal direction of the fibrous planes, and how the fibers are separated from each other at the grain boundaries 1. None of the coalescence planes of the twins in grain 3 is visible in FIG. 2.

FIG. 3 shows in perspective and greatly simplified a representation of a few fibrous grains of the polycrystal. The individual fibers 10 are substantially parallel to one another. Each individual fiber is composed of twin laminations whose corresponding coalescence planes 11 are identified by parallel lines along the fibers. The arrow 12 represents a 111 -direction of the fiber. The arrow 13 indicates the direction of growth, for example a 1l0 direction, of the entire crystal and is perpendicular to the arrow 12.

The above-described fibrous polycrystal according to the invention may be produced by the following method. A melt of the semiconductor material, such as silicon or germanium, is permitted to freeze from one end toward the other (normal freezing). The ingot thus produced is elongated, having the shape of a rod or bar. The melting and normal freezing operations may be performed in an elongated boat or crucible. After removing the solid ingot bar, a crystal seed is fused to the last solidified end. The seed crystal must be of fibrous structure similar to the one to be obtained in the ultimate product. That is, the seed itself is composed of elongated monocrystals having their longitudinal axes extend substar'itially parallel to each other. The seed need not be elongated in any particular direction but it must be fused to the bar of semiconductor material with the axes of the fibrous crystallites in the seed oriented longitudinally of the bar. As a rule, the fibrous crystallites in the polycrystalline seed are elongated twin-type monocrystals. After the seed is fused to the bar, the bar is subjected to zone melting commencing at the seed, the molten Zone progressing from the seed toward the other end of the rod at the usual speed.

If this method is performed by using a doped melt, it results in a homogeneously doped semiconductor poly crystal A device suitable for methods of the type just described is schematically shown in FIG. 4. Denoted by is a semiconductor bar or rod of the conventional polycrystalline structure. Fused to the rod is a crystal seed 16 having the above-mentioned fibrous structure. The seed is attached in a holding device 18. The fusion location is surrounded by a heating device 17, for example, a ringshaped radiator or of a copper coil energized by high frequency current. The heater 17 is displaceable longitudinally of the rod 15 as is indicated by an arrow. FIG. 4 particularly shows the stage of the method in which the seed 16 is fused to the rod 15 with the aid of the heating device 17. After this is done, the same heating device 17 is used to maintain a molten zone in the rod 15 and to displace this molten zone from the left to the right end of the bar with the result that the fiber structure of the seed is transferred to the rod.

For obtaining a homogeneous distribution of a doping substance that may be contained in the rod 15, all fibers of the seed 16 as well as the fibers in the semiconductor crystalline product obtained by the method described with reference to FIG. 4 must have substantially the same direction of growth. If the direction of growth in all of the respective fibers, as well as within the twin laminations of the individual fibers, is substantially the same, then, as already explained, a uniform distribution of the doping substance throughout the entire cross section of the crystal being converted by zone melting is secured.

Polycrystals having a fibrous structure according to the invention can be produced in a much simpler manner than monocrystals. This is so because with crystals according to the invention it is not necessary that the monocrystalline fibers, or their twin laminations located in the starting portion, i.e. in the first crystallizing portion of the rod, extended up to the opposite end or entirely through the last crystallizing portion of the rod. The fibers and their twin laminations may rather grow to any random extent. That is, during the crystallizing stage the growth of the individual monocrystalline fibers may terminate at any point, because each time new fibers having the same direction of growth are produced. Consequently, the polycrystalline products according to the invention may be given any desired length. This applies equally to elemental, compound and two-phase polycrystals.

The production, for example of an indium antimonide polycrystal having a fibrous structure with a given homo geneous doping, may be effected in the following manner. A melt of indium antimonide is produced in a carbonized boat of quartz, the melt in the boat being doped with the desired dopant and to the desired extent. The boat with the liquid melt is gradually pulled out of a furnace zone heated to a temperature above the melting point of the semiconductor material, the pulling being done at a suitable speed. Carbonizing the Wall of the quartz boat is advisable to prevent the InSb from baking onto the quartz. As the boat with the melt is thus being gradually and slowly removed from the hot furnace zone, the melt freezes from one end of the boat toward the other and converts to a polycrystal which may still contain crystallites of random shapes and crystallographically completely different orientations respectively.

After the entire melt is thus frozen to a solid bar, a crystal seed of the same material having a fibrous structure is placed against the last frozen end of the polycrystal and fused thereto. Thereafter the bar is zone melted at conventional speed commencing at the fusion locality and in a direction opposed to that of the original freezing. The longitudinal axes of the fibrous crystallites in the seed must be oriented in the direction of growth. That is, they must extend parallel to the longitudinal axis of the boat. In general such an orientation of the crystal seed can be secured without the aid of complicated auxiliaries, since the grain direction of the etched crystal in most cases is clearly discernible with the naked eye, although if desired a magnifying glass may be used.

The crystallizing speed during the normal freezing step of the process (first step) and the travel speed of the zone-melting operation in the opposite direction (second process step) are preferably so chosen relative to each other that any enrichment or depletion of doping sub stance, as may be contained in, or admixed to, the semi conductor material and will have migrated to one end of the rod at the termination of the first freezing step, is substantially compensated by the second method step which causes migration of such dopant substance in the opposite direction.

The very first seed of fibrous structure required for growing fibrous polycrystals according to the invention may be obtained from a polycrystal of suitable chemical composition which by chance exhibits a suitably fibrous crystal domain. Such domains are observed, for example, in polycrystals resulting from the first step of the above described manufacturing process, namely the normal freezing operation.

The fibers can be made visible, for example by etching. One of the suitable etching solutions is available in the trade under the designation CF-4. This solution may be applied diluted with water in the ratio 1:1. CP-4 is a. mixture of concentrated nitric acid, concentrated hydrofluoric acid, acetic acid and bromium, for example in the quantitative ratio of 200: 150: :3 cmfi.

Among the investigated cross sections were those of InSb fibrous polycrystals, sectioned perpendicularly to the direction of growth. Metallographic and X-ray (roentgenographic) methods showed that the cross section of the individual twin laminations varies between approximately 1n and a few mm. Laue reflection diagrams resulted in reflex images whose gravity points were definitely indicative of ll0 -directi0ns. Generally the maximal departure of the growing direction of individual twin laminations from the l10 -direction was smaller than 3 and went only in exceptional cases up to about 10.

The above-described method for producing the laminated structure of polycrystals according to the invention is analougously applicable not only for producing rods from a melt in a crucible, but also when applying a float ing-zone (crucible-free) zone-melting operation, also when performing the zone melting in a horizontal crucible or boat, when applying oriented freezing of any kind, and similar methods. In each case the molten material may be doped or not be doped, depending upon the requirements or desiderata of the particular case. The semiconducting material in a fibrous semiconductor polycrystal according to the invention may be constituted by any of the known semiconductor materials. Particularly suitable are the elemental semiconductors from the fourth group of the periodic system, for example silicon or germanium, also the above-mentioned semiconducting A B compounds such as indium antimonide, indium ersenide, gallium arsenide, and also the two-phase materials such as InSb-NiSb to be more fully described hereinafter.

The crystal orientations mentioned in this specification (and identified by rectangular or wavy brackets) are to be understood to represent only examples of all available equivalent orientations possible in accordance with the particular crystal-lattice symmetry. For example in lieu of the Miller Index 1l1 any of the following indicia I11 1T1 111 [T11], iii iii and m] may be substituted provided that the corresponding cor related orientations are applied in the analogous sense. The foregoing applies also if instead of the wavy brackets (relating to planes) round parentheses are employed. In this respect it should be noted that in the event a corn pound has a pronounced polarity, parallel but opposing directions (such as [111] and [131]) need not be identical with respect to crystal growing conditions. This may have an influence upon the choice of the most favorable crystal growing or zone travelling directions relative to the fiber structure to be produced in accordance with the invention.

As mentioned above, the invention applies also to fibrous polycrystals formed of two-phase materials. This will be more fully explained presently.

Semiconductor crystallize bodies eutectically composed of components of which one consists of semiconductor material and is quantitatively predominant, and the other is constituted by geometrically anisotropic inclusions of conducting material which are embedded in the first phase and extend substantially parallel to one another, are known as such. Reference may be had for example to the German application Zeitschrift fiir Physik, vol. 176, 1963, pages 399-408. A more detailed description and explanation may be found in the copending application of H. Weiss et al., Ser. No. 273,776, filed Apr. 17, 1963, now Patent No. 3,226,225, assigned to the assignee of the present invention. The material of such semiconductor bodies is here termed two-phase in analogy to the terminology employed in metallurgical phase diagrams relating to the melting points of compositions.

The term anisotropically geometrical shape is understood in this specification to mean that the inclusions have a preferred design or dimension in a given direction. For example, the inclusions may have the shape of needles or scales. If they are needle shaped, then the axes of the needles are substantially parallel to each other. If the inclusions have the shape of scales, then the scale faces are all substantially parallel to each other.

As a rule, the second phase, namely the anisotropic inclusions, consists of electrically good conducting material. However, they may also consist of magnetically good conducting material. An example of such a twophase material is a eutectic composition of indium antimonide (InSb) and nickel antimonide (NiSb), the NiSb forming needle-shaped inclusions of good electrical con ductance which are embedded in and dispersed throughout the bulk of a body consisting of InSb.

Semiconductor bodies of the two-phase type can be produced by melting semiconducting and good conducting material together in a eutectic quantitative ratio and thereafter subjecting the mixture to oriented solidification, preferably to normal freezing (freezing from one end toward the other of an elongated body of molten material) During freezing and growth of the crystal, the second phase segregates out and forms good conducting inclusions which, on account of the directional freezing, orient themselves preferably in a direction parallel to the temperature gradient at the solid-liquid boundary. Since, as mentioned, the inclusions are to be oriented in parallel relation to each other, it is important to take care that the solid-to-liquid boundary is planar and extends as accurately as feasible in a direction perpendicular to the crystal growing direction. Under such conditions, a temperature gradient can be formed at the liquid-to-solid boundary which extends parallel to the crystal growing direction at least on the average, taken over the entire boundary. Such a liquid-to-solid boundary can be maintained by suitable temperature control and observing a suitable crystallization rate at the growing front of the solidifying crystal, these expedients being known as such.

In general, the two-phase semiconductor bodies heretofore known have been produced in the form of ordinary polycrystals having granular crystallites of rand-om orientation and random shapes. These semiconductor bodies were cut out of bars or ingots produced by pulling a polycrystal out of a melt. In such crystalline ingots, however, only a central region of larger or smaller size is found to contain the good conducting inclusions with the desired, substantially parallel orientation relative to one another. In contrast thereto, the marginal regions of the ingots, amounting often up to 50% of the entire material, exhibit extreme disturbances of the required orientation of the inclusions. Consequently, disproportionately large amounts of material had to be discarded. Furthermore, even the orientation of the inclusions in the preferred central region of the ingot bar left much to be desired with respect to a number of purposes for which a finished semiconductor material was intended.

The mentioned, partially incomplete parallelism of the inclusions can be explained by the fact that the temperature gradient, determined as an average over the entire solid-to-liquid boundary, extends parallel to the direction of crystalline growth, but that the directions of the temperature gradient in the vicinity of each individual grain or crystallite of the growing polycrystal depart more or less greatly from one another,

Relating to two-phase semiconductor crystals, therefore, it is a more specific object of our invention to afford the production of such crystals which exhibit a higher degree of parallelism with respect to the distributed inclusions; and it is also an object to avoid or greatly minimize the occurrence of the above-mentioned losses of material in the production process.

It will be understood from the foregoing that the semiconducting material of the two-phase fibrous polycrystal constitutes a matrix for the embedded inclusions of the second, usually electrically good conducting phase, these inclusions being anisotropic and having their respective longitudinal axes extend substantially parallel to each other, the quantitative ratio of the embedding first phase or matrix to the second phase corresponding to the eutectic composition of the two substances.

Two-phase polycrystals according :to the invention can be produced in the same manner as described above, except that the semiconductor melt, prior to being subjected to normal freezing, is given an admixture of the above-mentioned second, good conducting phase which normally constitutes only a small quantity in comparison with the amount of the molten semiconductor material. Referring for example to a melt composed of a eutectic composition of indium antimonide and nickel antimonide, the two constituents are miscible when in the molten state. Thereafter the melt is subjected to normal freezing, joined by fusion with a seed crystal of fibrous texture as explained above and ultimately subjected to zone melting with the zone travel commencing at the seed and travelling away therefrom, also as described above. During normal freezing, the NiSb needles segregate out of the InSb because NiSb has no solid solubility in InSb.

Analogously, two-phase fibrous polycrystals according to the invention may also be produced by zone melting, floating (crucible-free) zone melting, pulling the crystal out of a melt, zone melting in a horizontal boat.

The inclusions thus occurring in the crystal, such as electrically good conducting needles of NiSb, have their longitudinal axes extend much more accurately parallel to the growing direction of the crystal than in the known two-phase crystals, That is, in two-phase crystals according to the invention, such as elongated bars or rods of this type, the anisotropic inclusions are uniformly oriented substantially throughout the entire crystalline body. Consequently, the yield of useful semiconductor slices or material from such rods is considerably increased, in many cases by 30 to 40%.

Referring to the schematic illustration of a section parallel to the direction of growth shown in FIG. 5 at a magnification of about 300:1, the orientation of the anisotropic inclusions within the polycrystal, otherwise similar to that represented in FIG. 1, will be readily apparent. As in FIG. 1, the grain boundaries in FIG. 5 are denoted by 1, the twin coalescence planes by 2. At some localities these planes are more crowded than in others. The fibrous grain 3 terminates in the illustrated section, whereas the fibrous grain is newly formed and occupies some of the space in which the fiber 5, now soon reaching its end, is located.

The good conducting inclusions of the two-phase crystal are constituted by needles 6 whose longitudinal directions are parallel to the direction of the twin-crystal coalescence planes 2 throughout the polycrystal, even at the grain boundary 1 or where the inclusions penetrate such boundaries. In some cases the inclusions are found to be more crowded at the grain boundary than in the middle portion of the individual fibrous crystallites.

The inclusions 6 which in FIG. 5 appear as straight lines, are represented in the cross section according to FIG. 6 by individual dots indicating the points where the needles intersect the cross-sectional plane. As will be seen from FIGS. 5 and 6, the inclusions are randomly but nevertheless fairly uniformly distributed throughout the entire crystal.

Represented in FIG. 7 is a diagram of the frequency of occurrence of the inclusions. The abscissa indicates the angular departure a (in angular degrees) of the actual direction of a number N of inclusions from the accurate orientation determined by the direction of crystalline growth. The ordinate of the diagram indicates the number N of inclusions appertaining to a given cross section of the semiconductor polycrystalline bar, which exhibit the angular departure a. The curves 21, 22 and 23 correspond to three cross sections transverse to the direction of growth taken at a mutual spacing of 1 cm. through a semiconductor bar of known two-phase structure.

For comparison, the curve 20 was determined in the same manner from a cross section of a two-phase poly crystalline bar according to the invention. As will be seen from curve 20, the maximum number of uniformly dis tributed inclusions in the polycrystalline bar according to the invention has been found to be always considerably higher than the maximum of any of the other curves taken with the known two-phase bars. In particular, the maximum of curve 20 may be about five times as high as the highest maximum of any of the curves taken with the known bars and represented, for example, by curves 21, 22 and 23. It has been found that the curve 20, indicative of the frequency of occurrence of a desired uniform orientation, remains virtually invariable for any other cross section of a polycrystalline bar having a fibrous structure according to the invention. This has been ascertained, for example, by measurements made with a bar of this kind having a length of 50 cm.

Measuring results of the kind represented in FIG. 7 definitely exhibit the superiority of fibrous polycrystals according to the invention over the known two-phase crystals. On the one hand, in the known two-phase crys tals, the orientation of the inclusions may greatly vary from centimeter to centimeter along the bar. On the other hand, the orientation of the inclusions also varies greatly within one and the same cross section so that the average departure from the median orientation determined over the entire cross section may amount to 10. In other words, a crystal according to the invention exhibits a substantially uniform orientation of the inclusions not only within a single fibrous crystallite but also throughout the entire cross section and over any cross section of the entire bar. In addition, the invention has the advantage that the average orientation of the inclusions is virtually identical with the prescribed direction of crystalline growth and that there are hardly any inclusions Whose orientation departs by more than 5 from the average direction. The inclusions in a two-phase crystal according to the invention, .therefore, have a very much better orientation than in the two-phase crystals as heretofore known.

As mentioned, two-phase fibrous polycrystals according to the invention are produced in the same manner as described above with reference to FIG. 4 except that the bar or rod is composed of two phases, preferably in the eutectic proportion. When performing the above-- described zone-melting step with such a bar, the fibrous structure of the seed 16 is imparted to the bar and simultancously the bar material, as it recrystallizes out of the travelling molten zone, is caused to form segregations consisting of the second phase, these segrcgations having the desired anisotropic shape and being oriented in parallel to the travel direction ofthe zone-melting operation.

If the two-phase material is not prepared from a molten mass of a eutectic composition, the polycrystalline bar formed of the melt will be differently composed at the first-freezing and last-freezing ends respectively. That is, depending upon the particular departure from the eutectic ratio of the constituent quantities, the first freezing por tion of the bar may consist virtually of semiconductor substance only, or it may consist virtually or predominantly only of the second-phase (inclusion) substance. In some cases, however, particularly for the production of electroluminescence diodes, it is desirable to intentionally produce this otherwise undesired effect.

A two-phase polycrystalline semiconductor may also be given a spacially homogenous doping in the manner described above. It is only necessary to correspondingly dope the material of the melt from which the production of the polycrystal is started.

The semiconducting first phase which forms the predominant constituent of a fibrous two-phase polycrystal according to the invention may consist of any available semiconductor material. Applicable are the same semiconductor substances as are mentioned above, namely the semiconducting elements from the fourth group of the periodic system and their compounds, for example silicon or germanium, as well as semiconducting A B compounds such as InSb, GaSb, InAs, GaAs.

Referring to semiconductor crystals of A B compounds, the inclusions of electrically good conducting material may consist of compounds of the type CB in which C denotes an element from the group Fe, Ni, Co, Cr, Mn; and B is an element from the fifth group of the periodic system. Examples of such compounds are the following systems: indium antimonide/nickel anti monide, indium antimonide/chromium antimonide, indium antimonide/iron antimonide, indium antimonide/ manganese antimonide, indium arsenide/chromium arsenide, indium arsenide/iron arsenide, indium arsenide/ cobalt arsenide, gallium antimonide/chromium antimo nide, gallium arsenide/chromium arsenide, and gallium arsenide/molybdenum arsenide. Also applicable are such systems as gallium antimonide/iron-gallium, or gallium antimonide/cobalt-gallium. In systems of the latter type the heavy metal combines with the component from the third group of the periodic system so that the resulting inclusions are of the type CB It should be noted that manganese antimonide is ferromagnetic. Hence the corresponding two-phase system, such as InSb/MnSb, is particularly well suitable for uses in which the ferromagnetism of the inclusions is of ad=- vantage. For example such a material is applicable for reflection filters having a pronounced polarization of the reflectivity. Also applicable with the semiconducting compounds indium antimonide and gallium antimonide are inclusions consisting of pure metals, for example antimony. If the semiconducting first phase consists of germanium, the inclusions may be formed by germanides of metals from the group Fe, Ni, Co, Cr, Mn.

As explained, the semiconductor material (first phase) and the inclusion material (second phase) in the starting melt of the production process should be generally applied in the eutectic mixing ratio. Examples of such eutectic mixtures of some of the above-mentioned systems are the following (all percentages being by weight): InSb and 1.8% NiSb, InSb and 0.67% FeSb, InSb and 6.5% MnSb, InSb and 0.6% CrSb; also GaSb and 13.4% CrSb, GaSb and 7.9% FeGa GaSb and 7.9% CoGa InAs 1 l and 10.5% FeAs, InAs and 1.7% CrAs; GaAs and 35.4% CrAs, GaAs and 5.3% Mo (as Mo-arsenide), GaAs and 8.4% VAs; InSb and 37.3% Sb, GaSb and 80.1% Sb.

Particularly advantageous is the above-mentioned eutectic composition of InSb with 1.8% NiSb, the representations in FIGS. 5, 6 and 7 being based on results obtained with this particular composition.

As mentioned, the inclusions, as a rule, are uniformly distributed within the polycrystal over its entire cross section and over its entire length. Inclusions of needle-shaped configuration, particularly those of NiSb, may have a thickness in the order of magnitude of 1 micron and a length in the order of 50 microns. However, greatly different dimensions of thickness and length have been observed, depending upon the specific properties of the particular two-phase system and also depending upon the freezing method employed. Thus, needle thicknesses of 0.1 up to 10 microns or more have been measured with different two-phase compositions. In general the needles may have a length ten times or more larger or smaller than mentioned above, although within one and the same crystal made by a single uniform method and employing a uniform rate of freezing, only slight variations about an average thickness and length of the needles have been observed.

If the inclusions are scale shaped, the requirement for substantially uniform orientation is tantamount to having the diameter of the scales large in comparison with the thickness and having the disc faces extend substantially parallel to each other. Relative to diameter and thickness of such scales, the relations are similar to those mentioned above with reference to needle-shaped inclu= sioris. The thickness of the scales may be in the order of magnitude of 1 micron, for example.

The mutual spacing between the inclusions in fibrous two-phase polycrystals according to the invention is comparable, as regards order of magnitude, to approximately the largest dimension of the inclusions. Consequently, the inclusions are very close to each other. This crowding of the good conducting inclusions and the freedom available withrespect to the shaping of the semiconductor bodies, affords producing very thin wafers, for example of 50 micron thickness, from the two-phase fibrous polycrystals. If such an individual wafer is traversed by electric current in the direction perpendicular to the preferred orientation of the inclusions, a high ohmic resistance of the wafer can be attained on account of its slight thickness.

Two-phase semiconductor crystals of fibrous constitution according to the invention are well suitable as magnetic-field responsive resistors, for example. For such purposes, the semiconductors, when in operation, are traversed by a current and simultaneously subjected to a magnetic field perpendicular to the flow direction of the current. It is then preferable to give the semiconductor crystal such an orientation that the direction of the inclusions (namely the longitudinal direction of the needles or the faces of the scales) extend perpendicularly to the current flow direction as well as perpendicularly to the direction of the magnetic field. In this manner, a maximal depend ence of the electrical resistance upon the magnitude of the effective magnetic field is secured. Two-phase fibrous polycrystals according to the invention are therefore particularly well applicable for the manufacture of components and devices for measuring, regulating or controlling magnetic fields, or for measur= ing, regulating or controlling positional changes by relative displacement of the semiconductor with respect to the magnetic field.

Two-phase fibrous polycrystals according to the in= vention, especially in the form of thin wafers, are also suitable for producing or sensing polarized light within the visible and invisible spectra. For such purposes the conducting inclusions, in totality, may be looked upon as forming a polarizing lattice of dipoles acting upon the electromagnetic waves. Depending upon the type of the inclusion material forming the electrical dipoles, and also depending upon the mutual spacing, the polarized light may be produced by reflection at the polycrystal accord= ing to the invention or also by passing light from a source through the crystal.

We claim:

1.' The method of producing a fibrous polycrystalline inorganic semiconductor body, which comprises melting a mass of inorganic semiconductor material having the shape of an elongated body, freezing the body progressively from one end to the other, fusing to the last-frozen end a polycrystalline seed of coalesced monocrystalline twins and having a fibrous texture formed by monocrystallites whose longitudinal directions are substantially parallel to each other and to the body and are of approximately the same crystallographic orientation, and zone melting the body starting at the seed to impart a similar fibrous texture to the resulting polycrystalline product 2. The method of producing a fibrous polycrystalline semiconductor body according to claim 1, wherein said semiconductor material when molten contains a doping substance for obtaining a doped fibrously textured poly crystalline product.

3. The method of producing a fibrous polycrystalline semiconductor body according to claim 1, wherein said mass of molten semiconductor material contains a quantitatively major semiconducting phase of a III-V material and a conducting phase segregating from the major phase during said freezing, whereby the resulting product forms a fibrous polycrystal with substantially uniformly oriented two-phase crystallites.

4. The method according to claim 3, wherein said molten mass is a eutectic mixture of said two phases.

5. The method according to claim 3, wherein said fibrous-polycrystalline seed is a two-phase composition corresponding to that of the product being produced.

6. A polycrystalline semiconductor body, consisting of fibrous elongated inorganic monocrystalline grains whose longitudinal axes are aligned in parallel with each other and are of the same crystallographic orientation, said semiconductor body having a eutectic structure of a semiconducting phase and inclusions of conducting material, from a second phase, said semiconducting phase constituting an A B compound, said inclusions having the approximate shape of needles aligned in parallel to one another and in parallel to the longitudinal axes of the grains, and said semiconductor body having twin monocrystalline grains, whereby the twin growth planes are positioned in parallel with the longitudinal axes of the grains.

7. The body of claim 6, wherein the semiconducting phase is InSb.

References Cited UNITED STATES PATENTS 3/1956 Pfann 23301 XR 7/1966 Folberth 252---518 XR US. Cl. X.R. 

